The LED lighting system is used more and more in current lighting devices. With the market demands and energy level regulation, dimmable LED drives with a high PF and high efficiency emerge. But the dimmable LED driving apparatus with a high performance on the market have the following problems more or less: a) a lot of control chips and complex external circuits are used to satisfy design requirements of LED driving; b) some dimmable drivers use a single stage PFC control chip, but flicker may appear thereby, and the LED will bear a significant amount of low frequency (100 Hz/120 Hz) ripple current, then, a big output capacitor is needed in order to re-duce the influence of the ripple current, which again increases the volume and cost of the entire driver and occupies a large structure space; c) the traditional BOOST PFC+DC/DC structure applied to the LED driving does not have a high efficiency, because an output therefrom is changed from a very high voltage (an output voltage from boost PFC is usually 400V) to a very low voltage. In addition, both PFC and second DC-DC need high voltage rated components, which increases the cost; d) the traditional averaging dimming will affect the optical effect and causes color temperature shift, and influences the LED luminescence quality; and e) an extensibility is lacked, and increasing new market demands, such as intelligent control and color mixing, can hardly be satisfied.
At present, there are a lot of dimmable LED driving systems on the market for solving related problems. For instance, the dimmable LED driving chip IW3610 of IWATT solves the problems of dimmer matching and frequent flicker using quite a few parts. This driving chip uses a BOOST PFC+flyback structure, but can neither balance the situation of efficiency and high PF value, nor realize a PWM dimming. Another solution uses a single stage flyback LED driver that may realize a high PF with a low cost, for example, the dimming LED driving chip LNK306PN of Power Integration and ICL8001 of Infineon. But the LED should bear a ripple current of commercial power frequency one or two times of the rated current, which seriously affects the LED performances and frequent flicker will easily occur in dimming.